A pacemaker is an electronic, medical device that provides electrical pulses to stimulate a patient's cardiac tissues. Each electrical pulse causes depolarization of the stimulated cardiac tissue, thus causing a heart contraction.
Pacing therapy is used to relieve the symptoms associated with certain types of abnormal heart rhythms. For example, a pacemaker may be indicated when a patient suffers from bradycardia--a condition in which the patient's heart rate is slower than is physiologically acceptable. In such a patient, the electrical pulses provided by the pacemaker serve to regulate the patient's heartbeat at a more acceptable rate.
The earliest pacemakers provided stimulation pulses at a constant rate to a single chamber of a patient's heart. The paced rate, which was set at the time of implantation, was usually a rate appropriate for the patient while at rest. If the rate had to be changed surgery was required because physical access to the pacemaker was needed to change the paced rate. Modern pacemakers provide telemetry systems for remotely reading and changing the paced rate and other pacemaker parameters.
The cardiac rhythm of a healthy person varies as a function of the person's metabolic needs. For example, when exercising a person's heart rate normally increases to keep up with the increased demands placed on the body. Because early pacemakers provided stimulation pulses at a fixed rate, pacemaker patients might suffer from fainting or lightheadedness if they exerted themselves too strenuously.
Rate-responsive pacemakers improved on fixed rate pacemakers by altering the paced rate when physiologically indicated. Typically, a base heart rate, appropriate for the patient at rest, is maintained by the pacemaker. A physiological sensor generating a signal indicative of the patient's metabolic needs is monitored by the pacemaker, which when needed, increases the paced rate above the base rate. For example, during physical exertion, a patient's metabolic need is greater, and the heart rate should be increased accordingly.
Many rate-responsive pacemakers contain a mechanical transducer, such as a piezoelectric crystal, to monitor a patient's level of activity. The piezoelectric crystal provides an electrical output when subject to mechanical stresses, such as may occur due to a patient's body movements during physical exertion. Unfortunately, some piezoelectric sensors are susceptible to providing erroneous indications of activity when subjected to vibrations.
Alternatively, physiological sensors may be used that rely on changes in, or absolute measurements of, a physiological parameter to indicate metabolic need. Ideally, the monitored parameter is readily measurable, responds quickly to changes in metabolic need, and is largely immune to outside influences. Much effort has been, and continues to be, expended in developing effective techniques and sensors for indicating a patients metabolic need. Possible parameters which may be measured as an indication of metabolic need include a patient's central venous temperature or blood oxygen saturation. Other sensors and techniques are disclosed in copending, commonly assigned U.S. patent application Ser. No. 08/259,084, filed Jun. 13, 1994, which is herein incorporated by reference.
A signal from the mechanical and/or physiological sensor is analyzed by the pacemaker to determine the patient's metabolic need. If the sensor signal exceeds a predetermined threshold, the pacemaker may determine that the patient's metabolic need has increased and increase the pacing rate accordingly. Interpolating values from a look up table may be used to determine a rate change or delta to be applied to the current paced rate to achieve a rate suitable for the measured metabolic need. Similarly, when the sensor signal falls below the threshold, the pacemaker may reduce the paced heart rate to a level appropriate for the lower level of patient activity. Thus, rate-responsive pacing attempts to provide a paced heart rate which increases and decreases in a natural way depending on the physiological needs of the patient.
For example, a pacemaker patient may begin to walk up several flights of stairs. If the patient had a constant rate pacemaker, it would continue to pace the heart at the programmed rate--a rate that may be inappropriate for the patient's level of activity, resulting in lightheadedness or fainting. However, if the patient had a rate-responsive pacemaker, the increased exertion (relative to the patient's resting state) would be detected by the pacemaker and the pacing rate increased accordingly. Similarly, when the patient ceases climbing the stairs, the pacemaker returns to pacing at the base rate, possibly gradually reducing the rate over a period of several minutes. Thus, the paced heart rate increases and decreases according to the metabolic needs of the patient.
Another advance in pacemaker technology was the development of the "demand" pacemaker. A demand pacemaker has the ability to sense a patient's natural cardiac activity, and delivers a pacing pulse only in the absence of such activity. Demand pacemakers use sensing leads to monitor the patient's intracardiac electrogram ("IEGM") to detect spontaneous cardiac depolarizations. Pacing pulses are then only provided if a depolarization is not detected within a predetermined time interval following a previous depolarization (either natural, or paced). Providing pacing pulses only when needed decreases the power consumption of the pacemaker, thereby lengthening the useful life of the pacemaker's battery. Furthermore, sensing natural cardiac activity reduces the possibility of providing stimulation pulses during a heart's T-wave, which is otherwise known to cause fibrillation.
In a healthy heartbeat, atrial and ventricular contractions are temporally related in a way that provides increased cardiac output. A major benefit of dual-chamber or universal pacemakers is the capability of providing a pacing regime that closely approximates the natural synchrony between atrial and ventricular contractions. In patients with severely compromised cardiac performance, optimization of hemodynamic performance may be critical to successful pacing therapy. A significant number of patients, who suffer from hemodynamically compromised cardiac output associated with CHF or cardiomyopathy may benefit from optimal chronotropic pacemaker stimulation.
Furthermore, a growing body of clinical evidence suggests that for some patients having severely compromised cardiac output may show improved hemodynamic performance after several weeks or months of chronotropic pacing therapy. In particular, patients suffering from hypertrophic cardiomyopathy, hypertrophic obstructive cardiomyopathy, and dilated cardiomyopathy have shown significant improvements, and seem most responsive to short A-V delays.
The dual-chamber pacemaker has the capability of providing chronotropic pacing therapy. It uses two or more leads, and may be programmed to pace and/or sense both chambers of the heart. For example, the pacemaker may be programmed to sense natural P-wave activity in the atrium, and to pace in the ventricle (P-V pacing). A universal pacemaker provides the medical practitioner a wide range of possible pacing regimes to consider when instituting pacing therapy for a cardiac patient.
When programming a universal pacemaker, the physician is thus faced with programming heart rates for resting and active conditions, but also with programming the lengths of various refractory periods, the desired A-V delay to maintain A-V synchrony, and whether to pace and/or sense either one or both chambers. Selecting the optimal combination of parameters and modes may prove difficult, especially since optimal pacing parameters and modes vary from patient to patient, and possibly from one pacemaker model to the next.
Adding to the difficulty of programming an optimal pacing regime, is the fact that pacemakers are programmed in an "open loop" manner. In other words, there is no direct and continuing feedback relating changes in cardiac performance to changes in the pacing regime.
In a typical scenario, the pacemaker is programmed at implant. During post-implant follow up visits, the physician may reprogram the pacemaker based on medical testing, as well as on patient input. However, subsequent changes in a patients cardiovascular condition may affect hemodynamic efficiency so that the pacemaker's programming no longer provides optimal pacing therapy. Since there is no feedback relating to cardiac performance, such suboptimal pacing might not be detected.
Suboptimal pacing in some patients may not only deprive the patient of the short term benefits of optimal pacing, but also the long term improvement in hemodynamic performance suggested by some clinical testing. Because of the open-loop nature of tuning programmable parameters of a pacemaker, whether rate-responsive or universal, it is difficult, if not impossible, to ensure near optimal pacing therapy. Therefore, to provide optimal pacing therapy, a "closed-loop" way of adjusting pacing parameters and evaluating cardiac performance is needed.